This application claims the priority of Korean Patent Application No. 10-2004-0095940, filed on Nov. 22, 2004, in the Korean Intellectual Property Office, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a flat display, and more particularly, to an organic electroluminescence display having an insulating layer selectively formed below a pixel electrode of each of red (R), green (G), and blue (B) sub-pixels.
2. Description of the Related Art
Organic electroluminescence displays include a plurality of pixels arranged in a matrix or array on a substrate, wherein each of the pixels includes R, G, and B sub-pixels. Each of the R, G, and B sub-pixels includes an electroluminescence (EL) unit including an anode electrode, a cathode electrode, an emissive layer provided between the anode and cathode electrodes, and a thin film transistor (TFT) driving the EL unit. As a voltage is applied across the anode and cathode electrodes, light emits from the emissive layer toward or away from the substrate, thereby displaying images.
In a conventional rear-type organic EL display emitting light from an organic emissive layer toward a substrate, light emitted from the organic emissive layer travels through the substrate via insulating layers, such as a protective layer, an interlayer insulating layer, a gate dielectric layer, and a buffer layer, which underlay the organic emissive layer. Therefore, different color light beams emitted from the organic emissive layer through the substrate have inconsistent chromaticity coordinates.
In an active matrix organic EL display using a TFT as a switching device, a plurality of pixels are arranged in a matrix on a substrate, wherein each of the pixels includes R, G, and B sub-pixels. Each of the R, G, and B sub-pixels includes one capacitor, an EL unit, and at least two TFTs, for example, a switching TFT and a driving TFT.
FIG. 1 is a sectional view of a conventional organic EL display including thin film transistors. In FIG. 1, for purposes of convenience, only organic EL units and driving TFTs in the R, G, and B sub-pixels, which are part of each pixel of the organic EL display, are illustrated.
Referring to FIG. 1, a substrate 100 includes an R pixel region 100R, a G pixel region 100G, and a B pixel region 100B. An R sub-pixel 10R is formed in the R pixel region 100R of the substrate 100, a G sub-pixel 10G is formed in the G pixel region 100G, and a B sub-pixel 10B is formed in the B pixel region 100B.
The R sub-pixel 10R includes an R EL unit and a TFT driving the R EL unit. The TFT includes a semiconductor layer 111, which is formed on a buffer layer 105 and has a source/drain region 112 and 113, a gate electrode 121 formed on a gate dielectric layer 120, and source/drain electrode 142 and 143 formed on an interlayer insulating layer 130 and connected with the source/drain region 112 and 113, respectively, via contact holes 132 and 133.
The R EL unit includes an anode electrode 161, which is a pixel electrode formed on a protective layer 150 and connected with the drain electrode 143 through a via hole 151, an organic layer 181 formed on the anode electrode 161 exposed by an opening 171 formed in a pixel isolating layer 170, and a cathode electrode 190 formed over or on a top surface of the substrate.
Similarly, the G sub-pixel 10G includes a G EL unit and a TFT driving the G EL unit. The TFT includes a semiconductor layer 114, which is formed on the buffer layer 105 and has a source/drain region 115 and 116, a gate electrode 124 formed on the gate dielectric layer 120, and a source/drain electrode 145 and 146 formed on the interlayer insulating layer 130 and connected with the source/drain region 115 and 116, respectively, via contact holes 135 and 136.
The G EL unit includes an anode electrode 164, which is a pixel electrode formed on the protective layer 150 and connected with the drain electrode 146 via a via hole 154, an organic layer 184 formed on the anode electrode 164 exposed by an opening 174 formed in the pixel isolating layer 170, and the cathode electrode 190 formed over or on a top surface of the substrate.
Similarly, the B sub-pixel 10B includes a B EL unit and a TFT driving the B EL unit. The TFT includes a semiconductor layer 117, which is formed on the buffer layer 105 and has a source/drain region 118 and 119, a gate electrode 127 formed on the gate dielectric layer 120, and a source/drain electrode 148 and 149 formed on the interlayer insulating layer 130 and connected with the source/drain region 118 and 119, respectively, via contact holes 138 and 139.
The B EL unit includes an anode electrode 167, which is a pixel electrode formed on the protective layer 150 and connected with the drain electrode 149 via a via hole 157, an organic layer 187 formed on the anode electrode 167 exposed by an opening 177 formed in the pixel isolating layer 170, and the cathode electrode 190 formed over or on a top surface of the substrate.
In a conventional organic EL display having the above-described structure, or a structure similar thereto, the protective layer 150 having a uniform thickness over the substrate underlies, e.g., is positioned beneath, the respective pixel electrodes 161, 164, and 167 of the R, G, and B sub-pixels 10R, 10G, and 10B.
However, in the above-described organic EL display, when the protective layer 150 having a uniform thickness is formed over the substrate, the chromaticity coordinate for B light shifts, thereby resulting in a narrower B chromaticity area, which is not suitable. In contrast, when the protective layer 150 is not formed, the chromaticity coordinate for B light may be suitable, however, the chromaticity coordinate for G light shifts, thereby unsuitably narrowing the G chromaticity area, which is not suitable.
U.S. Pat. No. 6,674,106 discloses an organic EL display that improves optical characteristics of light emitted from an organic emissive layer. The organic EL display includes a plurality of pixels arranged in a matrix on a substrate. The substrate includes an opening region, in which EL units acting as display units are arranged, and a non-opening region, in which thin film transistors for driving the EL units are arranged. By selectively removing the insulating layers underlying the pixel electrodes, i.e., regions of the gate dielectric layer and interlayer insulating layer corresponding to the opening region, which is a light emitting region, the refractive index in the opening region is adjusted to be nearly the same as the refractive index in the substrate, thereby improving the optical characteristics in the opening region.
In the above-described conventional organic EL display, the optical characteristics in the opening region may be improved by removing the regions of the gate dielectric layer and interlayer insulating layer that are aligned with the opening region. However, since the regions of the gate dielectric layer and interlayer insulating layer, which are aligned with the opening region and underlay the pixel electrode of each of the R, G, and B sub-pixels, are removed to allow light emitted from the organic layer to go through the substrate, an optical path cannot be controlled for individual R, G, and B sub-pixels.
Korean Patent Laid-open No. 2003-70726 discloses a rear emission type organic EL display in which a total thickness of insulating layers underlying an anode (pixel) electrode, such as a buffer layer, a gate dielectric layer, an interlayer insulating layer, a protective layer, etc., is controlled such that a chromaticity coordinate or light emitted from an organic emissive layer is improved, e.g., the chromaticity coordinate is optional.
In the organic EL display, for example, the chromaticity coordinate of light emitted from the organic emissive layer is optimal when the total thickness of the insulating layers underlying the anode electrode is approximately 2,500-3,500 Å. However, since the thickness of the insulating layers provided below the anode electrode is uniform over the substrate, light emitted from the organic layers of all the R, G, and B sub-pixels travels through the substrate via a common optical path. Therefore, the optical path cannot be controlled for individual R, G, and B sub-pixels.